With the rapid growth of the information industry and the continuous breakthroughs in the development of display technology, a trend is now becoming increasingly recognizable that flat panel displays (FPD), which take up much smaller space, are gradually taking over the place currently occupied by the traditional cathode ray tubes (CRT). Among the various flat panel displays, liquid crystal displays (LCD), because of their light weight, small thickness, low driving voltage required, and low energy consumption, have assumed a leading position. Another reason for the wide popularity of LCDs can also be attributed, at least in part, to the rapid development of the technologies that LCDs have been associated with. More recently, with the successful development of thin film transistors (TFT), LCDs now have acquired the capability of becoming a full-color display ready for a much broader consumer market. This development further enhances the already immense potential of LCDs.
With both the multi-color and full-color LCDs, chroma control and brightness control are the two most important elements. These elements are provided by a high-gray level (black-and-white) LCD, color filter films, and backlight devices. Of these elements, color filters provide the most important role for color control.
A color filter comprises three main components: a black-hued (i.e., black-colored) matrix, a color filter layer, and an overcoat. Currently, at least five methods have been disclosed in the prior art for making color filters. These include:
(1) dyeing, PA1 (2) etching, PA1 (3) pigment dispersion, PA1 (4) electrodeposition, and PA1 (5) printing.
The dyeing method and the etching method primarily utilize an appropriate arrangement of dyes to prepare color filters. A wide variety of dyes have been taught in the prior art references. Many of which provide homogeneous chroma, high dyeability, and allow a wide selection of compatible resins for which desired color intensity and light transmissibility can be provided. U.S. Pat. No. 4,820,619, the content thereof is incorporated herein by reference, a photosensitive composition is disclosed for use in preparing a color filter which contains a copolymer of glycidyl (meth)acrylate or glycidyl (.alpha.-methyl)vinyl ether with a (meth)acrylic amide or ester having a quaternary ammonium salt structure, and an aromatic azide as a photosensitizer. U.S. Pat. No. 4,837,098, the content thereof is incorporated herein by reference, discloses a colored filter layer comprises three groups of filter picture elements having spectral characteristics respectively corresponding to red, green, and blue. Each group of filter picture elements (R, G, B) are made of polyimide resin and dye contained therein.
Because of the relatively inadequate light and heat resistances of the dyeing materials, the methods of dyeing and etching discussed above have been largely replaced by the pigment dispersion method and/or the electrodeposition method, both of which utilize pigments that exhibit superior light and heat resistances. In these methods, pigment particles are uniformly dispersed in a resin matrix. Typically, the pigment particles are controlled to have a particle size less than 0.2 .mu.m so as to ensure reliable coloring characteristics. U.S. Pat. No. 5,085,973, the content thereof is incorporated herein by reference, discloses a color filter prepared by providing red, green, and blue image elements, each imaging element comprising a photosensitive resin and a pigment, and a black matrix on a transparent glass substrate. The photosensitive resin is formulated such that it comprises a polyfunctional acrylate monomer, an organic polymer binder and a photopolymerization initiator comprising a 2-mercapto-5-substituted thiadiazole compound, a phenyl ketone compound, and 2,4,5-triphenylimidazolyl dimer composed of two lophine residues combined to each other through intermediation of a single covalent bond. U.S. Pat. No. 4,786,148, the content thereof is incorporated herein by reference, discloses a color filter comprises a substrate and colored resin films, including blue, green, and red resin films containing blue, green, red colorant particles, respectively. The average particle volumes of the blue, green, and red colorants are set that the blue particles are greater than the green particles, which are further greater than the red particles. The pigment method is also disclosed in, for example, Japan Laid-Open Patent Publication JP60-129739. With the pigment dispersion method, lithographic techniques can be utilized to improve resolution, increase the flexibility of pattern design, and form color filters that can be used in TFT-LCDs. However, the conventional pigment-related methods typically involve a relatively complex process, and they require at least three photomasks which must be precisely aligned to ensure good quality. Furthermore, because the pigment dispersion method involves a free radical reaction to form a thermoset resin, a protective layer is required so as to avoid contact with oxygen.
With the electrodeposition coating processes, a transparent electrode is prepared by patterning a transparent electrically conductive film (typically an indium-tin oxide, or ITO) which is deposited on a substrate and serves as an electrode, and an electric voltage is applied only to a portion of the patterned transparent electrode which is to be dyed in the same color. The substrate is then immersed in a coloring electrodeposition bath containing appropriate polymers and pigment dispersed in water, and a colored layer is formed by electrodeposition. Thereafter, electric voltage is applied only to another portion of the substrate which is to be dyed in a different color, and the substrate is then immersed in another colored electrodeposition bath for forming a different color layer via electrodeposition. This procedure is repeated until all the desired colored layers are formed. The disadvantages of the electrodeposition coating process are that it is necessary to perform a high precision patterning of the transparent electrode, and to pay meticulous attention during the subsequent process not to break the fine pattern, because otherwise, the subsequent coloring process will be rendered very difficult. The electrodeposition coating processes typically are limited to the preparation of color filters for use in STN-LCDs.
Among all the processes for preparing color filters, the printing process is the least expensive process. However, it suffers the problems of having poor dimensional precision, smoothness, and reliability, and is not well accepted by the industry for making high quality electronic products.
By combining the pigment dispersion method and the electrodeposition coating methods, Nippon Oil Company proposed an electrodeposition lithographic method (ED-litho) for making color filters. In U.S. Pat. No. 5,214,542, the content thereof is incorporated herein by reference, Nippon Oil disclosed an electrodeposition lithographic method, which involves the steps of (a) forming a photosensitive coating film on a transparent electrically conductive layer provided on an outermost surface of a substrate having an alignment film, (b) exposing the photosensitive coating film to light through a mask having patterns of at least three different degrees of light transmittances; (c) developing and removing a photosensitive coating film portion registering with one of the patterns of smallest and largest degrees of light transmittances to expose the transparent electrically conductive layer; (e) electrodepositing a colored coating on the exposed electrically conductive layer to form a colored layer thereon, and (f) repeating the step (e) for the respective patterns of different degrees of light transmittances in sequence of difference in light transmittances to form different colored layers, respectively. U.S. Pat. No. 5,214,541, the content thereof is incorporated herein by reference, discloses the additional step of transcribing the colored layers, the transparent electrically conductive layer, and the alignment film onto another substrate.
The electrodeposition lithographic method discussed above has several advantages in that: (1) high precision patterns can be obtained, better than that obtainable from the electrodeposition coming method; (2) the pattern figure has a high degree of freedom, and both stripe and non-stripe patterns can be provided; (3) because it utilizes the advantageous characteristics of electrodeposition process, the coated films exhibit uniform film thickness and excellent smoothness.
In both the electrodeposition and the electrodeposition lithographic methods, the colored resins are electrically deposited on an electrically conductive layer formed on a transparent glass substrate (which is conventionally called an ITO layer, or simply ITO, although it could be made from a different material). As a result, the conductive part(s) of the ITO must be continuous in order to conduct current thereto. However, in order to accommodate the design of the colored patterns, the ITO must not be ubiquitously continuous, i.e., some portions of the ITO must be isolated from each other. During the assembly of the LCDs, the contacting surfaces must be extremely smooth. And the provision of an insulation layer in order to form these separate ITO regions can cause manufacturing problems.
First, a flat panel LCD screen is formed by superimposing the color filter with another flat glass panel with an adhesive layer typically arranged in the shape of a rectangular frame called "adhesive frame region". If the adhesive frame region has colored resin deposited thereon, because the mechanical strength of the colored resin is substantially weakened by the large amounts of pigments contained therein, the overall strength of the adhesion between the color filter and the flat glass panel will be degraded accordingly. Second, the flat glass panel and the color filter must closely parallel each other, and any unleveledness in the thickness of the adhesive layer would adversely affect this parallelness. The existence of any undesirably deposited colored resin on the adhesive frame region could cause the thickness of the adhesive layer to vary by as much as 2-3 .mu.m, thus severely breaching the stringent requirement on the leveledness of the adhesive frame and greatly affecting the quality of the LCD panel.
In order to ameliorate these problems, it has been suggested to form a solvent-soluable and/or mechanically-peelable insulation film, via an adhering or printing procedure, onto the ITO prior to the electrodeposition of the colored resins. After the completion of the electrodeposition process, the insulation film is then dissolved by an appropriate solvent, and/or is peeled off using a robotic arm. This process is dictated by the peelability of the insulation film, and it must be operated without causing any damage to the electrodeposited layers. Additionally, the extra step of peeling off the insulation film greatly complicates the manufacturing process and increases the manufacturing cost. The use of the solvent to peel off this insulation film also causes the clean room to be subject to an undesirably additional round of possible contamination, as well as causing an undesired solvent disposal problem that is inevitably associated therewith.
The insulation film can also be coated on the ITO using a screen printing device. However, the screen printing device typically does not meet the stringent in-line layout requirement of a high quality clean room, in addition to the environmental pollution problem associated with solvent evaporation. Furthermore, the resolution provided by a screen printing process does meet the level required in making color filters.